The epizootiology of Marek's disease 2. The effect of supply flock, rearing, house and production house on the incidence of Marek's disease

Mortality from Marek's disease (MD) was recorded between the ages of 8 and 22 weeks in a flock of about 8,000 female broiler breeders in an effort to determine the effect on mortality from MD of factors in the supply flock, rearing house and pen, and production house and pen. A single production flock derived from two supply flocks was chosen for study. Day old chicks were placed in pens in three rearing houses, and at 8 to 9 weeks of age half the chickens from each pen were placed in a pen in one house on a production site and the other half in a comparable pen in a second house. Statistical analysis of the results showed that only the rearing house pen of origin significantly affected the incidence of MD. Because infection with MD virus occurred in the rearing house it is suggested that the variable incidence of MD was due to a complex 'interrelationship between strains of virus of variable pathogenicity, resulting in natural vaccination by apathogenic strains prior to exposure to infection with pathogenic strains in some groups and not in others.